package com.hit.basmath.learn.others;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * 18. 4Sum
 * <p>
 * Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.
 * <p>
 * Note:
 * <p>
 * The solution set must not contain duplicate quadruplets.
 * <p>
 * Example:
 * <p>
 * Given array nums = [1, 0, -1, 0, -2, 2], and target = 0.
 * <p>
 * A solution set is:
 * <p>
 * [
 * [-1,  0, 0, 1],
 * [-2, -1, 1, 2],
 * [-2,  0, 0, 2]
 * ]
 */
public class _18 {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        ArrayList<List<Integer>> res = new ArrayList<List<Integer>>();

        int len = nums.length;
        if (nums == null || len < 4)
            return res;

        Arrays.sort(nums);

        int max = nums[len - 1];
        if (4 * nums[0] > target || 4 * max < target)
            return res;

        int i, z;
        for (i = 0; i < len; i++) {
            z = nums[i];
            if (i > 0 && z == nums[i - 1])// avoid duplicate
                continue;
            if (z + 3 * max < target) // z is too small
                continue;
            if (4 * z > target) // z is too large
                break;
            if (4 * z == target) { // z is the boundary
                if (i + 3 < len && nums[i + 3] == z)
                    res.add(Arrays.asList(z, z, z, z));
                break;
            }

            threeSumForFourSum(nums, target - z, i + 1, len - 1, res, z);
        }

        return res;
    }

    /*
     * Find all possible distinguished three numbers adding up to the target
     * in sorted array nums[] between indices low and high. If there are,
     * add all of them into the ArrayList fourSumList, using
     * fourSumList.add(Arrays.asList(z1, the three numbers))
     */
    private void threeSumForFourSum(int[] nums, int target, int low, int high,
                                    ArrayList<List<Integer>> fourSumList, int z1) {
        if (low + 1 >= high)
            return;

        int max = nums[high];
        if (3 * nums[low] > target || 3 * max < target)
            return;

        int i, z;
        for (i = low; i < high - 1; i++) {
            z = nums[i];
            if (i > low && z == nums[i - 1]) // avoid duplicate
                continue;
            if (z + 2 * max < target) // z is too small
                continue;

            if (3 * z > target) // z is too large
                break;

            if (3 * z == target) { // z is the boundary
                if (i + 1 < high && nums[i + 2] == z)
                    fourSumList.add(Arrays.asList(z1, z, z, z));
                break;
            }

            twoSumForFourSum(nums, target - z, i + 1, high, fourSumList, z1, z);
        }
    }

    /*
     * Find all possible distinguished two numbers adding up to the target
     * in sorted array nums[] between indices low and high. If there are,
     * add all of them into the ArrayList fourSumList, using
     * fourSumList.add(Arrays.asList(z1, z2, the two numbers))
     */
    private void twoSumForFourSum(int[] nums, int target, int low, int high,
                                  ArrayList<List<Integer>> fourSumList, int z1, int z2) {
        if (low >= high)
            return;

        if (2 * nums[low] > target || 2 * nums[high] < target)
            return;

        int i = low, j = high, sum, x;
        while (i < j) {
            sum = nums[i] + nums[j];
            if (sum == target) {
                fourSumList.add(Arrays.asList(z1, z2, nums[i], nums[j]));

                x = nums[i];

                // avoid duplicate
                while (++i < j && x == nums[i])
                    ;

                x = nums[j];

                // avoid duplicate
                while (i < --j && x == nums[j])
                    ;
            }

            if (sum < target)
                i++;
            if (sum > target)
                j--;
        }
    }
}
